Conventionally, exhaust gas recycling apparatus equipped with exhaust gas recirculating tubes are installed in internal combustion engines, while the exhaust gas recirculating tubes are used to recirculate a portion of the exhaust gas of the internal combustion engines from engine exhaust tubes to engine air intake tubes. This is such a system that since the portion of the exhaust gas of the internal combustion engines is mixed with sucked air, maximum combustion temperatures are lowered so as to reduce harmful substances (for example, NOx) which are contained in the exhaust gas.
Under such a circumstance, very recently, even in diesel engines, throttle valves (namely, intake throttles) have been started to be utilized in response to such a request for further improving exhaust gas performance in such a manner that a large amount of exhaust recycling gas (EGR gas) which is recirculated via exhaust gas recirculating tubes to engine air intake tubes is entered into the engine air intake tubes. The intake throttle valves control newly sucked air amounts in such a way that when valve bodies of exhaust gas recirculating amount control valves (EGR control valves) which adjust recirculating amounts (EGR amounts) of the EGR gas are opened, the newly sucked air amount may be reduced. Also, generally speaking, as actuators for performing open/close control operations of these intake throttle valves, DC motors operable in fast control response characteristics are employed in order to prevent occurrences of smoke during acceleration operation.
On the other hand, electronic throttle controller (electronic throttle systems) are known in this technical field, while these electronic throttle controllers drive DC motors in response to depression amounts of accelerator pedals so as to control opening degrees of throttle valves. In this electronic throttle controller, a drive current is supplied to a DC motor based upon an accelerator opening degree signal which is outputted from an accelerator position sensor for sensing a depression amount (accelerator opening degree) of an accelerator pedal. Then, since the DC motor is driven, opening/closing operations of the throttle valve are controlled in order that an amount of air sucked to a cylinder of an engine can become an optimum air intake value. Then, the DC motor is equipped with a throttle position sensor capable of sensing a throttle opening degree in order to perform a positional control of the throttle valve. Thus, a feedback control is carried out based upon the proportional integral and differential control (PID) control with respect to the DC motor in such a manner that deviation between a throttle opening degree signal (position sensor signal) outputted from the throttle position sensor and an accelerator opening degree signal outputted from the accelerator position sensor is reduced to become zero.
When an abnormal condition happens to occur in the throttle position sensor for detecting the throttle opening degree, the positional control operation of the throttle valve cannot be carried out under better conditions. For example, when the throttle position sensor is brought into malfunction indicative of a small opening degree (line interruption, short-circuit, output fixing), the throttle valve is opened more and more due to the feedback control, so that such a difficulty may occur. That is, output power of an internal combustion engine become excessively high. Also, for instance, when the throttle position sensor is brought into malfunction indicative of a large opening degree (line interruption, short-circuit, output fixing), the throttle valve is closed more and more due to the feedback control, so that such a difficulty may occur. That is, the internal combustion engine is brought into engine stall. In order to solve these difficulties, the energizing operation to the DC motor is immediately stopped, and the throttle valve is fixed to the mechanical predetermined opening degree by employing, for example, a default spring. Thus, output power of the internal combustion engine is limited, and the saving drive may be realized. However, even when the throttle position sensor is instantaneously brought into an abnormal condition, since the energizing operation to the actuator is immediately stopped, there is another problem that drivability as to a vehicle is deteriorated.
As a consequence, in order to solve such a problem, the below-mentioned electronic throttle controller is known in the technical field (refer to, for example, Japanese Laid-open Patent Application No. 2001-303976, pages 1 to 7, FIG. 1 to FIG. 4). In this electronic throttle controller, the energizing operation to the actuator is continued even when the abnormal condition happens to occur in the throttle position sensor until a predetermined time has elapsed; and when the abnormal condition of the throttle position sensor is continued for a time period longer than, or equal to the predetermined time, which is detected, the energizing operation to the actuator is stopped, and then, the throttle valve is fixed to the mechanical predetermined opening degree by using, for example, a default spring. As a result, when the abnormal condition of the throttle position sensor happens to occur, while the drivability is not deteriorated, this electronic throttle controller is capable of avoiding that the output power of the internal combustion engine becomes excessively high, and the internal combustion engine is brought into the engine stall condition. However, in this conventional electronic throttle controller, if the predetermined time is set to a long time duration, when the throttle position sensor is brought into malfunction indicative of a large opening degree (line interruption, short-circuit, output fixing), the throttle valve is closed more and more due to the feedback control, so that the internal combustion engine is brought into the engine stall condition.